1. Field of the Invention
This invention relates to phase stable, liquid aqueous enzyme-containing detergents, which have enhanced physical stability and improved enzyme stability.
2. Brief Description of the Prior Art
Liquid detergents are desirable alternatives to dry, granular detergent products. While dry, granular detergents have found wide consumer acceptance, liquid products can be adapted to a wide variety of uses. For example, liquid products can be directly applied to stains and dirty spots on fabrics, without being predissolved in water or other fluid media. Further, a "stream" of liquid detergent can be more easily directed to a targeted location in the wash water or clothing than a dry, granular product.
There have been many attempts to formulate liquid, aqueous detergents which include enzymes. Enzymes are very desirable adjuncts in liquid detergents since they are effective at removing stains which may not be cleaned through detergent or oxidative action. These problematic stains include grass stains and blood stains, which typically are complex mixtures of various substances such as proteins, fats, and natural coloring agents.
Attempts to compatibilize enzymes in these liquid, aqueous detergents, however, can lead to further problems, namely resulting physical instability. Many materials which act as enzyme stabilizers are insoluble in, or incompatible with, other components which make up typical liquid detergents.
Letton et al., U.S. Pat. No. 4,318,818, discloses a stable aqueous enzyme composition consisting of surfactants, pure enzyme, alcohol solvents, a lower carboxylate, a soluble calcium salt in an amount such that no more than 0.1-10 millimoles of calcium ion per liter are present, with the additional provisos that the pH is 6.5-10, but when the pH is 8.5 or less, only 2 millimoles of calcium per liter/water are present, and when the pH is greater than 8.5, the carboxylate must be a formate. Apparently, Letton recognizes that at low pH, the amount of calcium used as an enzyme stabilizer must be limited to very low amounts, or physical stability may be affected.
Kaminsky et al., U.S. Pat. No. 4,305,837, discloses stabilized aqueous liquid detergents containing surfactants, protease, alcohols, 3-10% of a water soluble formate, a soluble calcium salt which imparts 2-10 millimoles of calcium ion per liter, triethanolamine and water. Kaminsky concerns itself primarily with enzyme stability, and teaches that triethanolamine can be used for both enzyme stability and as a pH adjusting agent, which apparently buffers an aqueous solution to a pH of about 8.5-10.
Tolfo et al., U.S. Pat. No. 4,287,082, discloses a homogenous liquid detergent consisting essentially of a surfactant, a C.sub.12-14 saturated fatty acid, enzyme, a lower carboxylate selected from acetic acid, formic acid and sodium formate, and 0.5-1.5 millimoles of calcium ion per liter, in which the pH is 6.5-8.5. Tolfo, similar to its brethren patents, Kaminsky and Letton, above, discloses that very small, discrete amounts of calcium ion should be present in these liquid compositions in order to stabilize the enzymes.
In each of the above three references, the amount of calcium ion present must be rigorously limited. The explanation for this is that high amounts of calcium can be precipitated by anionic surfactants which may be present in the liquid compositions. Note, for instance, that Tolfo, which specifically recites the presence of saturated fatty acid, has the lowest amount of calcium ion present, namely, 0.5-1.5 millimoles.
Severson, U.S. Pat. No. 4,537,707, discloses a liquid detergent containing anionic surfactants, fatty acids, builders, protease, boric acid, water-soluble formate, 1-30 millimoles of calcium ion/liter and water. Severson teaches a boric acid/formate complex ". . . which effectively cross-link[s] or staple[s] an enzyme molecule together, thereby holding it in its active spatial confirmation." (Column 2, Lines 51-53). Thus, Severson focuses on enzyme stability by the use of a boric acid/calcium/formate complex.
Hughes, U.S. Pat. No. 4,507,219, discloses a liquid detergent containing an alkyl or alkenyl sulfonate, an alkyl ethoxylated sulfate, a nonionic surfactant, a mixed C.sub.10-14 saturated fatty acid, a water soluble polycarboxylate builder, a source of potassium and sodium ions in a molar ratio of 0.1:1.3 K:Na, ethanol, polyol and water. The invention claimed by Hughes is an unusual neutralization system comprising mixed potassium and sodium ions, which are necessary to maintain the homogeneity of the polycarboxylates in the aqueous dispersion. Thus, Hughes contemplates a complex system where a specific builder is maintained in solution by means of an involved neutralization procedure.
Barrat, U.S. Pat. No. 4,111,855, discloses liquid detergents in which a polyacid and a source of calcium ions forms water-soluble calcium complexes in order to enhance enzyme stability. Thus, this reference contemplates the use of polycarboxylic acids in conjunction with calcium ions solely for enzyme stability.
Inamorato et al., U.S. Pat. No. 4,368,147, discloses a liquid detergent consisting essentially of C.sub.10-18 alkoxylated nonionic surfactant, having 2-3 moles of alkoxy per mole of alcohol, a C.sub.2-3 alcohol, a sodium or potassium formate as a viscosity control and gel preventer, and the balance, water. This particular patent does not employ enzymes, and thus avoids problems introduced when enzyme stabilizers, such as calcium salts, are added to liquid detergents.
Kebanli, U.S. Pat. No. 4,490,285, discloses a nonionic surfactant, C.sub.10-18 alcohol ethoxysulfate, which is apparently monoethoxylated, and the ratio of nonionic:alcohol ethoxysulfate is about 2:1 to 4:1, and a solvent system comprising water or mixtures thereof with C.sub.1-6 alcohol. This patent is restricted to the use of an alcohol monoethoxysulfate. The inventions claimed in Kebanli are contended to be superior in detergency over polyethoxylated sulfates.
Review of the foregoing prior art reveals that none of these references disclose, teach or suggest that alkyl ether carboxylates are effective phase stabilizers for liquid detergents containing enzymes stabilized by calcium ions. The prior references all focus on individual problems in the field. Thus, most of the references discuss enzyme stability in a liquid system (Letton, Tolfo, Kaminsky, Severson, Barrat), while others concern themselves with stabilization of the various, somewhat incompatible materials in the liquid system (Inamorato, Hughes), while still others focus on materials which assertedly impart improved detergency (Kebanli). None of the foregoing references seeks to achieve both good phase stability and enzyme stability of liquid detergents by the use of effective amounts of alkyl ether carboxylates.